Vertical shaft impact crushers (VSI-crushers) are used in many applications for crushing hard material like rocks, ore etc. WO 2004/020103 describes a VSI-crusher comprising a housing and a horizontal rotor located inside the housing. Two separate material flows may be fed to the crusher. A first material flow is fed to the rotor via an opening in the top thereof. The first material flow is accelerated by the rotor and is ejected towards the wall of the housing. A second material flow is fed outside the rotor, for example, between the rotor and the housing. This second material flow is hit by the first material flow ejected by the rotor. Thus the first and second material flows are crushed against each other just outside the rotor.
In some situations the operation of the crusher described in WO 2004/020103 may be disturbed by problems in the feeding of the second material flow. Such problems reduce the crushing efficiency of the crusher and increase the need for maintenance work.